Si la stands for 希臘,. in Mandarin: [ɕi˥˥ la˥˩]. in Middle Chinese: [hiəi lɑp]. in Hokkien Chinese: Hi-láh [hi˨˨ laʔ˥]. and in original ancient Greek: Hellas [he'las]. So, the logic is: one Chinese character stands for a single syllable, while Hel-las has two syllables, it would be best to translate it into a two-character word. read more
Because Greece is called Greece in English. It is actually the name Hellas in Greek. Chinese translate a country's name in their native primary language, in this case Greek, into phonetic equivalent in Chinese, hence the tramslated name 希腊, or xi la。Another example is Germany. read more